The state of the art comprises, for the firing or the drying of manufactured articles which are mainly made of ceramic material, so-called tunnel kilns, in which the products to be subjected to heat treatment are loaded onto containers made of refractory material on board trolleys which pass through the kiln, the firing cycle of the material taking place at predetermined times and rates. It also comprises kilns in which the material is loaded onto motorised transverse rollers which cross the kiln for the transport of the same material in the longitudinal direction, the external ends of each roller being rotatably supported close to the lateral walls of this kiln.
DE 35 15 856 discloses a tunnel kiln having two opposite rows of transport rollers which pass through the opposite side walls of the kiln; the internal ends of said rollers being equipped with wheels lying on the floor of the kiln, in order to support and transport in a longitudinal direction a pair of longitudinal beams hold between flanges protruding circumferentially from said wheels; said longitudinal beams supporting a number of cross members in order to constitute a loading frame for the items to be treated in the kiln.
Moreover, in the tunnel kilns, considerable thermal power is used for the heating of the trolley, with a consequent reduction of the relative thermal efficiency; furthermore, each trolley, which consists of an expensive metal structure covered in refractory material, needs frequent maintenance operations with regard on the one hand to the restoration of the covering, which tends to break up as a result of the thermal shocks and of the inevitable knocks during movement, and on the other hand to the necessity of frequent greasing of the hubs of the wheels, to avoid their deterioration and the consequent seizing.
As far as the rollers are concerned, these become encrusted through contact with the wet and/or green glaze material, causing obstacles to the flow of the material, with possible harmful moving across and thickening of this towards the walls, avoidable only by means of periodic thorough cleaning of the rollers, to be carried out by the previous extraction of the same through the walls of the kiln, which, employing considerable labour, necessitates large spaces for manoeuvring at the side of the kiln and great financial outlay, the latter also being contributed to by the breaking of rollers which occurs during the cleaning phase, especially in the case of rollers of considerable length which have modest carrying capacity.
Another disadvantage, in the case of manufactured articles which are short in relation to the interval of the rollers, derives from the need to support them by the interposition of plates which are subject to harmful deviations in trajectory, particularly in the case of encrusted rollers.
Furthermore, the width of the kiln is limited by the maximum length which can be assigned to the rollers; this latter, which is currently not greater than 3.4 m, does not in fact permit the firing of products of large dimensions, which, however, is desirable.
In the kiln according to DE 35 15 856 the wheels fastened at the internal ends of each roller rotate in contact with the floor of the kiln thus generating a frictional tangential force resulting in a lateral bending stress and a torsion stress on each roller and in an increase of drive torque for each roller; particularly when heavy items are to be conveyed.
Furthermore, the sliding friction between the wheels and the floor of the kiln causes a considerable loss of energy and a rapid wearing of the floor of the kiln in the contact areas between the floor and the wheels, which can result in an unbalanced distribution of the loads on the rollers due to a misalignment of the axes thereof.
This state of the art is susceptible to considerable improvements with regard to the possibility of avoiding the disadvantages indicated above. What has been stated above gives rise to the need for a solution to the technical problem of obtaining a kiln for the firing or the drying of manufactured articles, in particular of ceramic material, which are also of considerable mass and dimensions, which kiln permits considerable reductions in consumption, waste and maintenance, in order to achieve a great reduction in costs, such a kiln also having to allow both the method of firing manufactured articles by continuous unidirectional passing through, and that by alternating or intermittent flow; the mechanical wearing of the floor of the kiln being also eliminated.